Diagnosing student learning barriers has been recognized as the most fundamental and important issue for improving the learning achievements of students. In the past decade, several learning diagnosis approaches have been proposed based on the concept–effect relationship (CER) model. However, past studies have shown that the effectiveness of this model heavily depends on the concept relationship knowledge provided by the domain experts (eg, experienced teachers or educators for a specified subject); ie, the performance of the developed learning diagnosis systems could be significantly affected by subjective opinions, ignorance or insufficient knowledge if those concept relationships are derived from a single domain expert. To cope with this problem, this study proposes a group decision approach for developing the CER model with the cooperation of multiple domain experts. Based on the proposed approach, a testing and diagnostic system has been implemented; moreover, an experiment has been conducted to evaluate the effectiveness of this new approach. The experimental results show that this approach is able to develop quality CER models, and hence the low-achievement students who received the generated learning suggestions had significantly better learning achievements than those who learned with the previous approach.